The invention relates to devices and methods for measuring the absolute polarization of alkali metal atoms wherein the operation of a detection laser is improved by controlling the laser operating temperature and/or current. The polarized alkali metal atoms are contained within a sample cell disposed within magnetic fields, and the intensity of the detection laser light after passage through the sample cell is measured to determine the polarization.
Recent developments in magnetic resonance tomography (MRT) and in magnetic resonance spectroscopy (NMR) with polarised inert gases can be expected to yield applications in medicine, in physics and in materials sciences. High nuclear spin polarisation levels in inert gases can be achieved by optical pumping using alkali metal atoms, as can be seen from the paper by Happer et al., Phys. Rev. A, 29, 3092 (1984). Typically at present, the alkali metal atom rubidium is used in the presence of an inert gas and nitrogen. In this way, it is possible to achieve a nuclear spin polarisation of ca. 20 percent in the inert gas xenon (129Xe). Such a nuclear spin polarisation is ca. 100,000 times greater than the equilibrium polarisation in clinical magnetic resonance tomographs. The consequent drastic increase in the signal-to-noise ratio explains why in the future new possible applications are expected in medicine, science and technology.
Polarisation is understood to mean the degree of alignment (ordering) of the spin of atomic nuclei or electrons. For example, 100 percent polarisation means that all nuclei or electrons are oriented in the same way. A magnetic moment is associated with the polarisation of nuclei or electrons.
Polarised xenon is for example inhaled by a person or injected into him. 10 to 15 seconds later, the polarised xenon collects in the brain. Using magnetic resonance tomography, the distribution of the inert gas in the brain is established. The result is used for further analyses.
The choice of the inert gas depends on the particular application. 129Xenon displays a large chemical shift if xenon is for example adsorbed on a surface, then its resonance frequency changes significantly. Furthermore, xenon dissolves in fat-loving (i.e. lipophilic) liquids. When such properties are desired, xenon is used.
The inert gas helium is almost insoluble in liquids. The isotope 3He is therefore regularly used when cavities are concerned. The lungs of a person are an example of such a cavity.
Some inert gases have valuable properties other than the aforesaid. Thus for example the isotopes 83Krypton, 21Neon and 131Xenon have a quadrupole moment, which is for example of interest for experiments in fundamental research, namely in surface physics. However, these inert gases are very expensive, so that these are unsuitable for applications in which larger amounts are used.
From the paper xe2x80x9cB. Driehuys et al., Appl. Phys. Lett., 69, 1668 (1996), the polarisation of inert gases in the following way is known.
Using a laser and, a xcex/4 plate positioned in the light beam from the laser, circularly polarised light is produced, that is to say light in which the angular momentum i.e. spin of the photons all point in the same direction. The angular momentum of the photons is transferred to the electrons of alkali metal atoms. Hence the spins of the electrons of the alkali metal atoms display a large deviation from the thermal equilibrium. Consequently, the alkali metal atoms are polarised. As a result of a collision of an alkali metal atom with an atom of an inert gas, the polarisation of the electron spin of the alkali metal atom is transferred to the nuclear spin of the inert gas. Polarised inert gas is thus produced.
Alkali metal atoms are used as these have a large optical dipole moment, which interacts with the light. Further, alkali metal atoms each have one free electron, so that no disadvantageous interactions between two and more electrons per atom or molecule can arise.
Caesium would be a particularly suitable alkali metal atom, which Is superior to rubidium for the production of polarised xenon. However, at present there are no lasers available with sufficiently high power, such as would be needed for the polarization of xenon using caesium. It is however to be expected that in the future lasers with power levels of about 100 watts at the caesium wavelength will be developed. Probably caesium will then be preferentially used for the polarisation of inert gases.
The state of the art is that a gas mixture at a pressure typically of 7 to 10 bars is slowly passed through a cylindrical glass cell. The gas mixture consists 98 percent of 4Helium, one percent nitrogen and one percent of xenon. The typical flow rates for the gas mixture are a few cc per second.
The gas mixture first flows through a vessel (hereinafter termed xe2x80x9cfeed vesselxe2x80x9d) which contains ca. one gram of rubidium. The feed vessel with the rubidium present in it, together with the glass cell connected to it, is heated to ca. 100 to 150 degrees centigrade. By the provision of these temperatures, the rubidium is vaporised. The concentration of the vaporised rubidium atoms in the gas phase is determined by the temperature in the feed vessel. The gas flow transports the vaporised rubidium atoms from the feed vessel into the cylindrical sample cell. A powerful, circularly polarised laser (100 watts power in continuous operation) irradiates the sample cell, which is generally a glass cell, axially and optically pumps the rubidium atoms into a highly polarised state.
Here, the wavelength of the laser must be matched to the optical absorption line of the rubidium atoms (Dl line) In other words: in order optimally to transfer the polarisation of light to an alkali metal atom, the frequency of the light must coincide with the resonance frequency of the optical transition. The sample cell is located in a static magnetic field B0 of a few tens of Gauss, which is created by coils, in particular a so-called Helmholtz coil pair. The direction of the magnetic field runs parallel to the cylinder axis of the sample cell, i.e. parallel to the direction of the laser beam. The magnetic field serves to control the polarised atoms.
The rubidium atoms optically highly polarised by the light of the laser collide in the glass cell inter alia with the xenon atoms and give up their high polarisation to the xenon atoms. At the exit of the sample cell, the rubidium is deposited on the wall, owing to its high melting point compared to the melting points of the other gases. The polarised xenon or the gas mixture is passed on from the sample cell into a freezing trap. This consists of a glass flask, the end of which is immersed in liquid nitrogen. The glass flask is moreover located in a magnetic field with a strength of 1000 to 2000 Gauss. The highly polarised xenon gas is deposited as ice on the inner glass wall of the freezing trap. At the outlet of the freezing trap, the remaining gas (helium and nitrogen) is passed through a needle valve and finally released.
The flow rate in the whole apparatus can be controlled with the needle valve, and measured with a gauge. If the flow rate increases too much, no time remains for the transfer of the polarisation from the rubidium atoms to the xenon atoms. Hence no polarisation is achieved. If the flow rate is too low, then too much time elapses before the desired amount of highly polarised xenon has been frozen. Thus the polarisation of the xenon atoms again declines through relaxation. The relaxation of the xenon atoms is greatly retarded by the freezing and by the strong magnetic field, to which the freezing trap is exposed. Hence it is necessary to freeze the inert gas as quickly and with as little loss as possible after the polarisation. The relaxation admittedly cannot be avoided by the freezing. However, there still remain 1 to 2 hours before the polarisation has declined so much that a subsequent use of the initially highly polarised gas is no longer possible.
A polariser of the aforesaid type always has joints. Joints are places at which at least two pipes through which the polarised gas is passed are joined together. The pipes as a rule consist of glass. The joint is created by a connecting element, such as e.g. flanges.
The light of the laser, which creates the polarisation, is absorbed in the sample cell. The intensity of the light and hence the polarisation of the alkali metal atoms in the sample cell decreases correspondingly. For technical reasons, the cross-section of the sample cell is not in general uniformly illuminated by the light of the laser. Consequently, the alkali metal atoms are not uniformly polarised. Interactions with the walls of the sample cell likewise alter the polarisation of the alkali metal atoms along the cross-section of the sample cell. Consequently, the polarisation of the alkali metal atoms in the sample cells varies depending on location.
For the control and analysis of the polarisation of inert gases, it is necessary to measure this as a function of the location in the sample cell. From the paper S. Appelt et al., Phys. Rev. A 58, 1412 (1998) and from A. Ben-Amar Baranga et al., Phys. Rev. A 58, 2282 (1998), measurement of the absolute polarisation of the alkali metal atoms as a function of location as follows is known.
RF coils are mounted on both sides of the sample cell. By means of these coils, an oscillating magnetic field is created in the sample cell. The oscillating magnetic field created by the RF coils is overlaid by the static magnetic field B0 created by the Helmholtz coils. The magnetic field lines of the static magnetic field run parallel to the longitudinal axis of the cylindrical sample cell. The magnetic field lines of the oscillating magnetic field of frequency; xcfx89RF run perpendicular to this. The interaction of the two magnetic fields in the sample cell has the result that a precessing cone of the electron spin polarisation of the alkali metal atoms arises when the frequency of the RF magnetic field coincides with the Larmor frequency of the total spin of the rubidium atom.
The magnetic field B0 is continuously varied, so that the cone arises on attainment of the Larmor frequency, and disappears again when the Larmor frequency is left. Alternatively, the frequency of the RF magnetic field could be correspondingly scanned at constant B0 field.
For the detection of the cone, a titanium-sapphire laser, which creates circularly polarised light, irradiates the sample cell perpendicularly to the longitudinal axis. This light interacts with the transverse component of the cone. The light absorption is dependent on the presence of the cone. A photodetector is mounted on the opposite side of the sample cell in such a way that it measures the light of the laser. The photodetector xe2x80x9cseesxe2x80x9d the rotating transverse component of the cone as a modulation signal with the frequency xcfx89RF The signal of the photodetector is demodulated, so that the signal is then described by a resonance curve. With a low field B0, the resonance curve shows only one resonance frequency in the form of one peak. For large magnetic fields B0, that is magnetic fields typically of 30 Gauss and over, a large number of resonance frequencies can be measured. From this signal, the absolute polarisation of the alkali metal atoms is determined.
In order to be able to measure the absolute polarisation of the alkali metal atoms along the cross-section of the sample cell perpendicular to the magnetic field lines of the B0 field (in the x-direction), a gradient coil is also provided, with which a gradient in the B0 field is created in the sample cell. By this means, location information is encoded: the polarisation can then be determined as a function of the x-direction of the sample cell.
The device has the disadvantage that the polarisation can only be measured as a function of location along the x-direction. Also disadvantageous is the fact that a very expensive titanium-sapphire laser must be used. The use of an inexpensive laser diode was hitherto found to be impossible.
An object of the invention is the creation of an improved measurement device of the type mentioned at the outset for measurement of the absolute polarisation of alkali metal atoms in the sample cell.
The invention has particular application in a polarizer for inert gas wherein alkali metal atoms transfer polarization to the inert gas and polarization of the alkali metal atoms is laser detected. The disclosed apparatus and methods of operation provide improved polarization measurements.
The device includes a sample cell and also in one form of the invention means of polarizing alkali metal atoms in the sample cell. By means of Helmholtz coils (coil pair), the B0 magnetic field, whose magnetic field lines run along a direction which is referred to hereinafter as the z-direction, is created in the sample cell. RF-coils are provided, which create an oscillating magnetic field perpendicular to the z-direction. This direction is referred to hereinafter as the x-direction. The circularly polarized light of a laser, hereinafter also referred to as the detection laser, shines through the sample cell in the x-direction. By means of a sensor xe2x80x94in particular a photodetectorxe2x80x94the intensity of the light after passing through the sample cell is measured, the measured signal is fed into an electronic processor and thus the polarization of the alkali metal atoms in the sample cell is determined.
In one embodiment of the invention, a laser can irradiate the sample cell in the z-direction. It,the light of the laser is previously circularly polarised with a xcex/4 plate, then the alkali metal atoms in the sample cell are polarised.
Means for controlling the temperature of the detection laser are provided. In particular, the means include a temperature sensor, with which the temperature of the detection laser is measured. Further, the means include a heating/cooling device, with which the detection laser can be heated or cooled as required. The heating/cooling device is controlled by a control device depending on the measured temperature of the detection laser. By the aforesaid means, the temperature of the laser is kept constant.
It has been found that the maintenance of a constant temperature results in improved measurement results. Further, a sufficiently constant temperature is a prerequisite in order to be able to use an inexpensive semiconductor diode as detection laser instead of a titanium-sapphire laser. In particular, improved measurement results are obtained with the use of the semiconductor diode.
In one form of the invention, the means for the maintenance of a constant temperature are so designed that the temperature fluctuations are not more than one thousandth of a degree centigrade per hour.
In order to keep temperature fluctuations at the laser as small as possible, in a further form of the invention the laser is embedded in a heat-conducting metal, in particular in copper. The heat capacity of the metal is then very much greater than the heat capacity of the laser. Temperature fluctuations of the laser are cushioned by the metal. In this way, it is possible to maintain the desired high constancy of temperature. It has been found that the temperature constancy is an essential measure for achieving good measurement results. In particular, the volume of the metal should be several times greater than the volume of the laser.
In a further form of the invention, a Peltier element is used to heat or cool the laser as required. In order to function perfectly, the Peltier element is then in particular in contact with a heat sink. The heat sink generally has so-called cooling fins, which ensure a large surface area and hence rapid removal of heat.
In a further form of the invention, an electronic system is provided, which keeps the current with which laser provided for the detection of the alkali metal polarlsation is operated constant. This supply current for the laser should in particular fluctuate by less than 10 ppm. A suitable electronic system, which fulfils the requirements, is described in the paper xe2x80x9cRev. Sci. Instrum. 61 (8), August 1990xe2x80x9d. Through this further form of the invention, the measurement result is further improved.
In a further form of the invention, a semiconductor diode, in particular a xe2x80x9cmono mode laser diodexe2x80x9d is used as the laser for the detection of the polarisation of the alkali metal atoms as a function of location. Such a laser is considerably cheaper than the titanium-sapphire laser used in the state of the art. The price difference amounts at present to a factor of ca. 100 to 1000. Further, it has been found that with the laser which consists of a semiconductor diode better results compared to a titanium-sapphire laser are obtained, when the temperature and the supply current are stabilised in the aforesaid manner. The amplitude noise of the laser consisting of the semiconductor diode is then very much less than the amplitude noise of a dye- or a titanium-sapphire laser. The frequency noise of the laser consisting of the semi-conductor diode is similar to the frequency noise of a non-stabilised titanium-sapphire laser, if the aforesaid measures for the stabilisation of the temperature and the supply current are taken. Because of the improved amplitude noise, better measurement results compared to the state of the art are obtained. A semiconductor diode is small and light compared to a titanium-sapphire laser. Even with the use of a metal block which has a much greater heat capacity than the diode, the assembly is small and light compared to a titanium-sapphire laser. The assembly according to the invention with the semiconductor diode is thus especially mobile, which is of especial advantage in a particular application mentioned below.
In a further form of the invention, the semiconductor diode is a single (mono) mode laser diode with a power of for example ca. 20 milliwatts. By this choice, the frequency stability is advantageously ensured. The wavelength at which the semiconductor diode emits light is in particular about 795 nanometres.
In a further form of the invention, the semiconductor diode together with the detector lying opposite is mounted onto a carriage displaceable in the z-direction. By displacement of the carriage relative to the sample cell, the polarisation of the alkali metal atoms in the sample cell is in addition measured in the z-direction. A two-dimensional picture of the absolute polarisation of the alkali metal atoms is thus obtained.
The carriage can be a plate rolling on rails, on which are mounted the semiconductor diode together with the detector or sensor lying opposite and the further optical elements, when necessary, such as lenses, mirrors, and linear or circular polarisers. Further, in particular, the gradient coil is mounted on such a carriage. This enables rapid and uncomplicated measurement of two-dimensional pictures of the rubidium polarisation.
The measurement results enable optimal adjustment and control of the polarisation device, in order to polarise inert gases as efficiently as possible.
In a further advantageous form of the invention, the device has gradient coils which extend over a considerably greater volume (for example over twice as great a volume) than the volume of the sample cell. By this means, it is possible to provide an almost linear gradient (dB0/dx=constant) of the B0 magnetic field. Then not only can the measurement signal be evaluated in a particularly simple manner, but in addition it is also ensured that unambiguous assignment of polarisation to a location x is possible.